Trick Duke: Queen not attending formal function (9)
I believe the answer is:
deception
'trick' is the definition.
(synonyms)
'duke queen not attending formal function' is the wordplay.
'duke' becomes 'd'.
'queen' becomes 'r' (r is abbreviation regina. e.g. in ER ).
'not attending' suggests deleting specific letters.
'formal function' becomes 'reception' (I've seen this before).
'reception' with 'r' taken away is 'eception'.
'd'+'eception'='DECEPTION'
(Other definitions for deception that I've seen before include "Duplicity" , "Fraud" , "Trick" , "Illusion" .)